In general, surface plasmon resonance refers to a phenomenon having a light at a certain incident angle (the resonance angle) hits a thin metal film so that the light reflected from the film drops to a minimum intensity. However, light in the sample medium cannot naturally excite surface plasmon resonance and a high refractive index-prism or grating is required. When a light beam is incident through the prism on the surface of the thin metal film at total reflection angle, the evanescent wave interacts with the sample, and at the resonance angle, it couples with the surface plasmon. Hence, a fraction of the incident light energy transfers to surface plasmon resonance and the energy of the reflected light diminishes. The resonance angle is extremely sensitive to the dielectric permittivity at the interface. Thus, surface plasmon resonance can be used for biospecific interaction analysis.
A surface plasmon resonance sensing system is sensing system made in accordance with the aforementioned surface plasmon resonance phenomenon. Since a surface plasmon resonance sensor is sensitive to the local refractive index change at the metal/sample interface, it is not necessary to label an analyte molecule with a spectroscopic or electrochemical signature, and thus the surface plasmon resonance sensors possess the advantages of label-free and real-time detection, short analysis time, and high sensitivity. It has been applied extensively for detecting biological molecules.
A free electron cloud on the metal nanoparticle surface is excited by an electromagnetic field with a specific frequency to produce a collective dipole resonance, but the oscillating electron cloud is restricted in the neighborhood of nanoparticles, and thus such a resonance is called a localized plasmon resonance (LPR). It is interesting to find that if the environmental refractive index around the metal nanoparticles is changed, the frequency and the extinction cross-section of the LPR band will be changed accordingly. If the environmental refractive index around the metal nanoparticles increases, the peak wavelength of the LPR band will shift to a long wavelength and the extinction cross-section of the LPR band will increase. While observing the characteristic of a scattered light, we may find that when the refractive index of the medium rises, the peak wavelength in the spectrum of the scattered light also shifts to a long wave and with an increase of the light intensity.
In recent years, the development of nano materials has become a main subject for researchers and manufacturers, and the industries such as optoelectronics, communications and medical instruments spend a lot of effort on the research and development of the nano materials. A primary reason of the nano materials becoming favorable materials resides on that the nano materials provide properties totally different from the characteristics of the original sample. In the prior art, noble metal nanoparticles are used to excite the localized plasmon resonance (LPR) to substitute the traditional way of using noble metal films to excite the surface plasmon resonance (SPR) so as to improve the sensitivity and other analytical performance features (e.g., ease of miniaturization, simplicity in construction, and cost) of the sensor. At present, the technology of synthesizing nanoparticles is well developed, and basically divided into chemical and physical methods. The physical methods include a metal vaporization method, a laser etching method and a sputtering method, etc, and the metal vaporization method is the most commonly used one among these methods. The chemical methods include a reduction method and an electrolysis method, etc, and the reduction method is the most commonly used and important one. However, technologies and science advance rapidly, the requirement for the sensitivity of the sensors becomes increasingly higher, and thus it is an important subject for related researchers and manufacturers to improve the sensitivity and analytical performance of the sensor.